White Dwarfs: Compact Corpses of Stars White Dwarfs: Compact Corpses of Stars By Nola Taylor Redd, Space.com Contributor | November 26, 2013 06:11pm ET MORE NASA's Hubble Space Telescope captures a field of stellar husks. These ancient white dwarfs are 12 to 13 billion years old, only slightly younger than the universe itself. In theory, white dwarfs will eventually stop emitting light and heat and become black dwarfs. Credit: NASA and H. Richer (University of British Columbia). The stars in the sky may seem ageless and unchanging, but eventually most of them will turn into white dwarfs, the last observable stage of evolution for low- and medium-mass stars. These dim stellar corpses dot the galaxy, leftovers from brightly burning stars. Formation Main-sequence stars form from clouds of dust and gas drawn together by gravity. How the stars evolve through their lifetime depends on their mass. The most massive stars, with eight times the mass of the sun or more, will never become white dwarfs. Instead, at the end of their lives, they will explode in a violent supernova , leaving behind a neutron star or black hole .

Smaller stars, however, will take a slightly more sedate path. Low- to medium-mass stars, such as the sun , will eventually swell up into red giants , eventually shedding their outer layers into a ring known as a planetary nebula (early observers thought the nebulae resembled planets such as Neptune and Uranus). The core that is left behind will be a white dwarf, a husk of a star in which no hydrogen fusion occurs. Smaller stars, such as red dwarfs, don't make it to the red giant state. They simply burn through all of the hydrogen within the star, leaving behind the shell that is a white dwarf. However, red dwarfs take trillions of years to consume their fuel, far longer than the 13.8-billion-year-old age of the universe, so no red dwarfs have yet become white dwarfs. Characteristics When a star runs out of fuel, it collapses inward on itself. White dwarfs contain approximately the mass of the sun but have roughly the radius of Earth. This makes them incredibly dense, beaten out only by neutron stars and black holes. The gravity on the surface of a white dwarf is 350,000 times that of gravity on Earth.

White dwarfs reach this incredible density because they are so collapsed that their electrons are smashed together, forming what is called "degenerate matter." This means that a more massive white dwarf has a smaller radius than its less massive counterpart. Burning stars balance the inward push of gravity with the outward push from fusion, but in a white dwarf, electrons must squeeze tightly together to create that outward-pressing force. As such, having shed much of its mass during the red giant phase, no white dwarf can exceed 1.4 times the mass of the sun. This image is NGC 6543 known as the Cat's Eye Nebula as it appears to the Chandra X-Ray Observatory and Hubble Telescope. A planetary nebula is a phase of stellar evolution that the sun should experience several billion years from now, when it expands to become a red giant and then sheds most of its outer layers, leaving behind a hot core that contracts to form a dense white dwarf star. This image was released Oct. 10, 2012.

Credit: X-ray: NASA/CXC/RIT/J.Kastner et al.; Optical: NASA/STScI One last kick While many white dwarfs fade away into relative obscurity, eventually radiating away all of their energy and becoming a black dwarf, those that have companions may suffer a different fate. If the white dwarf is part of a binary system, it may be able to pull material from its companion onto its surface. Increasing the mass can have some interesting results. [ VIDEO: Cannibal White Dwarf Feeds on Companion Star ] One possibility is that adding more mass to the white dwarf could cause it to collapse into a much denser neutron star. A far more explosive result is the Type 1a supernova . As the white dwarf pulls material from a companion star, the temperature increases, eventually triggering a runaway reaction that detonates in a violent supernova that destroys the white dwarf. This process is known as a single-degenerate model of a Type 1a supernova. If the companion is another white dwarf instead of an active star, the two stellar corpses merge together to kick off the fireworks. This process is known as a double-degenerate model of a Type 1a supernova.

At other times, the white dwarf may pull just enough material from its companion to briefly ignite in a nova, a far smaller explosion. Because the white dwarf remains intact, it can repeat the process several times when it reaches the critical point, briefly breathing life back into the dying star over and over again. Author Bio Nola Taylor Redd, Space.com Contributor Nola Taylor Redd is a contributing writer for Space.com. She loves all things space and astronomy-related, and enjoys the opportunity to learn more. She has a Bachelor’s degree in English and Astrophysics from Agnes Scott college and served as an intern at Sky & Telescope magazine. In her free time, she homeschools her four children. Follow her on Twitter at @NolaTRedd Nola Taylor Redd, Space.com Contributor on Latest on White Dwarfs: Compact Corpses of Stars Texas Astronomer Solves Mystery of 'Born Again' Stars with Hubble Space Telescope | McDonald Observatory Texas Astronomer Solves Mystery of 'Born Again' Stars with Hubble Space Telescope 7 December 2015 by Rebecca Johnson AUSTIN — University of Texas astronomer Natalie Gosnell has used Hubble Space Telescope to better understand why some stars aren’t evolving as predicted.

These so-called “blue stragglers” look hotter and bluer than they should for their advanced age. It’s almost as if they were somehow reinvigorated to look much younger than they really are. Though blue stragglers were first identified 62 years ago, astronomers have yet to converge on a solution for their odd appearance. The most popular explanation among several competing theories is that an aging star spills material onto a smaller companion star. The small star bulks up on mass to become hotter and bluer while the aging companion burns out and collapses to a white dwarf – a burned out cinder. To test this theory Gosnell’s team conducted a survey of the open star cluster NGC 188 that has 21 blue stragglers. Of those, she found that seven had white dwarf companions, by identifying their ultraviolet glow that is detectable by Hubble. Of the remaining 14 of the 21 blue stragglers, a further seven show evidence of so-called mass transfer between stars in other ways. Gosnell said she believes these are older white dwarf-blue straggler binaries, and indicate two-thirds of blue stragglers form through mass transfer.

“This was really great,” Gosnell says. “Until now there was no concrete observational proof, only suggestive results,” Gosnell said. “It’s the first time we can place limits on the fraction of blue stragglers formed through mass transfer.” This discovery sheds light on the physical processes responsible for changing the appearance of 25 percent of evolved stars. Gosnell’s work, which closes gaps in our understanding of how stars age, is published in the current issue of The Astrophysical Journal. The problem came to light because in recent years, astronomers have been able to make a complete and accurate census of stars in a number of open star clusters, Gosnell said. “Open clusters really are the best laboratory for the study of stellar evolution,” Gosnell said. “They have a simple stellar population.” The stars in a cluster form at the same time and from the same materials, she explained. The cluster population studies revealed that up to a quarter of the oldest stars “are not evolving like we think they’re supposed to,” Gosnell said. Stars that astronomers expected to become red giants (like Aldebaran, the eye of Taurus, the bull) instead became “blue stragglers,” unexpectedly bright, blue stars with a host of strange characteristics.

Gosnell wanted to find out what happened to them. So she, along with Bob Mathieu at the University of Wisconsin-Madison and their collaborators, designed a study using Hubble Space Telescope’s Advanced Camera for Surveys to try to differentiate between three theories of how these stars became blue stragglers. The theories included: collisions between stars in the cluster (with debris coalescing to form a blue straggler), the merger of two of the stars in a triple star system, or mass transfer between two stars in a binary pair. In a binary pair of stars, the larger star will evolve faster, Gosnell said. That star becomes a red giant. A red giant is so bloated that the outermost layers of gas on its surface are only tenuously held by the star’s gravity. They can be pulled off by the gravity of the companion star. This is mass transfer. As the gas is siphoned off by the partner, the red giant is left with only its core, making it into a white dwarf. The partner — initially the less massive of the pair, but now the heavier one — becomes a blue straggler.

Gosnell’s method is limited by the fact that it will not detect white dwarfs that have cooled down enough so that they don’t glow in UV light detectable by Hubble, she said. That means that only those white dwarfs formed in the last 250 million years (youngsters, astronomically speaking) are detectable. Knowing more about how these stars form is important because astronomers use their assumptions to model the stellar populations of distant galaxies (where the light from all the stars blends together). “You don’t want to be ignoring 25 percent of the evolved stars” in those galaxies, Gosnell said. Such models are important because distant galaxies figure into many different types of cosmological studies. Right now, Gosnell said, “the models have a lot of room for improvement.” “If we tweak the way models treat mass transfer, that would bring the observations and theory together,” Gosnell said. “They would agree. And we can use this to inform our understanding of unresolved stellar populations” — that is, those stars in galaxies so far away that all their light is blended together. Gosnell plans to continue studying these stars using the 2.7-meter Harlan J.

Smith Telescope at McDonald Observatory and its IGRINS spectrograph to constrain the number of blue stragglers that could form through mergers in triple systems. — END — What Different Types Of Stars Are There WHAT DIFFERENT TYPES OF STARS ARE THERE PRINTABLE SPACE PUZZLE WHAT DIFFERENT TYPES OF STARS ARE THERE? I know of 10 different types of stars. They are called neutron stars, black dwarfs, white dwarfs, brown dwarfs, red dwarfs, yellow dwarfs, blue giants, red giants, blue supergiants and red supergiants. A neutron star is the remaining star after a giant star explodes and goes supernova. The neutron star can now have up to three times the mass of our sun and it is packed into only a small ball with a radius of around 6 to 10 miles or 10 to 15 kilometers. A neutron star is so dense that on earth just one teaspoon full would way a hundred million tonnes! A black dwarf is the stage after a white dwarf. It takes so long for stars to get to this stage that none has probably ever gotten to this stage.

Our universe is so young and it will take a few times older than our universe even is for any sun to turn into a black dwarf. Our nearest star, Proxima Centauri is a red dwarf and scientists believe that it will take billions of years for it to end it's life cycle and eventually become a white dwarf and then a black dwarf. Black dwarfs are stars that are burnt out and do not have any light or heat anymore. Our Sun will probably become a black dwarf too someday. A white dwarf only has the radius of 0.02 of the Sun and it's true brightness is 0.005 of the Sun. It's surface temperature is 45,000 degrees Fahrenheit or 25,000 degrees Celsius. A white dwarf is not really white, its color depends on its temperature, and it can be from violet to deep red. A white dwarf can not be seen without a telescope. Did you know a teaspoon full of white dwarf would way 5 tonnes? A white dwarf is one of the last stages of a star from when the giant star releases its outer layers of gases and leaves the center or core of the star.

The white dwarf is then only about the size of the Earth but still has 3/4 the mass of our Sun. A brown dwarf is when a star was being created but it didn't get enough mass to become a real star. It is a bit bigger than Jupiter or a white dwarf but smaller than our sun, but they actually have about 13 to 75 times Jupiter's mass. It has a radius of 0.1 of the Sun and has a true brightness of 0.00001 of the Sun. It's surface temperature is 1,300 degrees Fahrenheit or 720 degrees Celsius. Instead of staying the same size or expanding it will shrink and cool down and fade. The brightest brown dwarf sometimes resembles a red dwarf. A red dwarf's radius is half of our Sun's radius and its true brightness is 0.05 of the Sun. A red dwarf is a low mass star, and since small stars take very long to slowly burn up there fuel they last a very very long time. A red dwarf can live for more than 10 times the present age of our universe, around 200 billion years at its regular rate.

It's surface temperature is 6,000 degrees Fahrenheit or 3,300 degrees Celsius. Red dwarfs are the most popular type of sun and make up just over half of all the stars in our entire galaxy! If our Sun was a red dwarf, from earth it wouldn't provide us with daylight it would only give us light like when our sun is rising or setting. Our nearest neighbour star Proxima Centauri is a red dwarf and it can't be seen in the sky because red dwarfs are usually not bright enough to be seen without a telescope. It is part of a triple star system. A yellow dwarf is a star like our Sun. A yellow star lives around 10 billion years. Eventually a yellow star will bloat into a red giant and it's outer layer will shoot out into space leaving a white dwarf and then later a black dwarf. A yellow's sun's radius and true brightness is 1 of our Sun. Its surface temperature is 10,000 degrees Fahrenheit or 5,500 degrees Celsius. For more information about yellow stars you can look at the section called All About Our Sun. I drew this picture on my computer.

A blue giant's radius is 10 Suns and its true brightness is 800,000 suns. Its surface temperature is 80,000 degrees Fahrenheit or 45,000 degrees Celsius. A blue giant is a star that was born a giant and later in its life cycle just like a yellow star it will bloat into a bigger star but instead of turning into a red giant it turns into a red supergiant. A blue giant usually lives a pretty short life. A red giant's radius is hundreds of times larger than the Sun and its surface temperature is relatively low. The surface temperature is around 8,540 degrees Fahrenheit or 4,727 degrees Celsius. A red giant is a star that increase in temperature and implodes and the heat energy it releases causes it to expand. A red giant is a star like our Sun after it expands and cools down and gets a bit dimmer. Once our Sun becomes a red giant it will turn all of our inner planets into cinder. A blue supergiant is an expanded blue giant.

They have a surface temperature of 36,000 to 90,000 Fahrenheit or 20,000 to 50,000 degrees Celsius. There radius can be up to 25 of our Sun. They are rare and some of the brightest stars in the universe. As the blue supergiant's core reaction speeds up and slows down and the wind changes can change back and forth from a blue supergiant and a red supergiant. A blue supergiant has fast but rare wind compared to a red supergiant which has a slow wind. Blue supergiants are rarely found with older stars they are found with young stars. A red supergiant has a radius of 500 Suns and a true brightness of 40,000 Suns. It has a surface temperature of 6,100 degrees Fahrenheit or 3,400 degrees Celsius. They are the biggest in size but not in mass or surface temperature or true brightness in the known universe. A red supergiant ends its life it in a super nova. A red supergiant is a star that is in its last phase of its life burning helium.

A red supergiant is just a bigger version of a red giant. Red supergiants have no photosphere. I drew this picture on my computer. White Dwarfs, White Dwarf Stars Information, Facts, News, Photos -- National Geographic Space Photograph courtesy NASA/Tod Strohmayer (GSFC)/Dana Berry (Chandra X-Ray Observatory) When they reach the end of their long evolutions, smaller stars—those up to eight times as massive as our own sun—typically become white dwarfs. These ancient stars are incredibly dense. A teaspoonful of their matter would weigh as much on Earth as an elephant—5.5 tons. White dwarfs typically have a radius just .01 times that of our own sun, but their mass is about the same. Stars like our sun fuse hydrogen in their cores into helium. White dwarfs are stars that have burned up all of the hydrogen they once used as nuclear fuel. Fusion in a star's core produces heat and outward pressure, but this pressure is kept in balance by the inward push of gravity generated by a star's mass. When the hydrogen used as fuel vanishes, and fusion slows, gravity causes the star to collapse in on itself.

As the star condenses and compacts it heats up even further, burning the last of its hydrogen and causing the star's outer layers to expand outward. At this stage, the star becomes a large red giant. Because a red giant is so large, its heat spreads out and the surface temperatures are predominantly cool, but its core remains red-hot. Red giants exist for only a short time—perhaps just a billion years compared with the ten billion the same star may already have spent burning hydrogen like our own sun. Red giants are hot enough to turn the helium at their core, which was made by fusing hydrogen, into heavy elements like carbon. But most stars are not massive enough to create the pressures and heat necessary to burn heavy elements, so fusion and heat production stop. Further Incarnations Such stars eventually blow off the material of their outer layers, which creates an expanding shell of gas called a planetary nebula. Within this nebula, the hot core of the star remains—crushed to high density by gravity—as a white dwarf with temperatures over 180,000 degrees Fahrenheit (100,000 degrees Celsius).

Eventually—over tens or even hundreds of billions of years—a white dwarf cools until it becomes a black dwarf, which emits no energy. Because the universe's oldest stars are only 10 billion to 20 billion years old there are no known black dwarfs—yet. Estimating how long white dwarfs have been cooling can help astronomers learn much about the age of the universe. But not all white dwarfs will spend many millennia cooling their heels. Those in a binary star system may have a strong enough gravitational pull to gather in material from a neighboring star. When a white dwarf takes on enough mass in this manner it reaches a level called the chandrasekhar limit. At this point the pressure at its center will become so great that runaway fusion occurs and the star will detonate in a thermonuclear supernova. White Dwarfs White Dwarfs White dwarfs are the burned-out cores of collapsed stars that, like dying embers, slowly cool and fade away. They are the remnants of low mass stars, among the dimmest objects observable in the Universe.

They are low to medium (less than ten solar mass) Main Sequence stars which have burned through their reservoirs of both hydrogen and helium, passed through the giant phase, were not hot enough to ignite their carbon, puffed off their outer layers to form colorful planetary nebula, and then collapsed and cooled into small glowing coals. This beautiful Hubble Space Telescope image shows a nearby white dwarf, and the outer layers of the former star's atmosphere which have been blown away. The resultant planetary nebula will shine for the next 20,000 to 50,000 years, expanding outwards and fading slowly with time. Nearby planetary nebula NGC 2440, surrounding a piping hot white dwarf with a surface temperature of 200,000 degrees Celsius. [NASA/HST, H. Bond, R. Ciardullo] A typical white dwarf has a carbon and oxygen mass similar to the Sun, but is much smaller in size (similar to the Earth). It is much hotter (25,000 K), but because of its small size its luminosity is low. How then can we find them?

The nearby globular cluster M4, observed from the ground (left) and with the Hubble Space Telescope (right). [NOAO/KPNO, M. Bolte & NASA/HST, H. Richer] Some very nearby white dwarf stars can be observed directly through telescopes, though they are extremely faint. M4, shown above, is the nearest globular cluster to the Earth. It contains hundreds of thousands of stars visible with ground-based telescopes, and is expected to contain about 40,000 white dwarfs. This globular cluster formed early in the history of the Milky Way, and today is a veritable stellar retirement community. It is so ancient (14 billion years old) that all of its stars that began with 80% or more of the Sun's mass have already evolved off of the Main Sequence to become red giants, and many have turned into white dwarfs. In the Hubble Space Telescope picture shown above, the brightest of the detected white dwarfs (the faint pin-pricks circled on the right hand side of the figure) is no more luminous than a 100-watt light bulb seen at the Moon's distance (239,000 miles).

A thin layer of hydrogen is transferred from the red giant, and slowly accumulates about the surface of the white dwarf. When the hydrogen builds up to a critical level, it triggers a supernova 1a explosion. [NMSU, N. Vogt & NASA/STScI] We can also observe the presence of a white dwarf in a binary system through its effect on its stellar companion. Consider a solar system composed of a white dwarf and a Main Sequence star. For a long, long time, the two stars will orbit each other in harmony. But eventually, the Main Sequence star will run out of hydrogen in its core and begin to swell up into a red giant. At some point, the star will balloon out to such a large radius that the outer atmosphere becomes more gravitationally attracted to the white dwarf than the star, and it will begin to drift towards it. The white dwarf star slowly accumulates a layer of hydrogen on its surface (see a beautiful simulation of this process by John Blondin). When about 10-8 solar masses of hydrogen has been accumulated, the temperature and pressure at the base of this layer will be great enough so that thermonuclear reactions begin (just like in a stellar core).

The hydrogen quickly begins to fuse into helium. This reaction is explosive, and ejects the burning layer of gas into space. Under certain conditions the result is a type of supernova (a type 1a supernova). With such a large mass concentrated into such a small size, we can deduce that white dwarfs are extremely high density objects. In a normal Main Sequence star, the pressure of gravity is withstood by the forces of nuclear fusion. In a white dwarf, however, all nuclear fuel has been exhausted and gravity compresses the core inwards, forcing the matter into a degenerate state. We find ionized carbon and oxygen, and a sea of electrons which have been forced out of the atoms. Degeneracy pressure (by degenerate we mean that the electrons and the protons have separated) may sound complicated, but it is actually a familiar process. It is no more than the pressure that prevents liquids and solids from being compressed. Gas pressure is squishy, and gases are compressible, as you can verify by pumping your bicycle tire with a hand pump. At a density of a few grams per cubic centimeter, however, ordinary matter becomes liquid or solid and is nearly incompressible.

Squeeze on any solid object, and it will push back. No apparatus on Earth can compress water (or steel) to half its volume. The same degeneracy pressure prevents atoms from collapsing within white dwarfs. Moreover, the tendency of liquids and solids to resist compression is almost independent of their temperature. Liquids and solids do not shrink much when cooled, even to almost absolute zero temperature (-273 C). Contrary to our usual experience, if we could heat a balloon of degenerate gas it would not expand. This has very important consequences for thermonuclear reactions, such as those that occur in stellar cores. Because of this, a white dwarf is able to radiate heat and slowly cool down, without the internal pressure dropping. (Contrast this with the pressure due to the heat and excitation of nuclear reaction when the star is in its Main Sequence hydrogen-burning phase: without the heat, the pressure drops and gravity wins the battle to compress the star.) The white dwarf comes slowly to the end of its life story. It has no nuclear hydrogen or helium left to burn and it will have no further major structural changes.

Because of its small size, high density, and initially hot temperature, it takes billions of years for a white dwarf to radiate all of its residual heat into space. It will take many millions of years before it cools off enough that it becomes invisible (a black dwarf). As it cools, the carbon atoms will crystallize into diamond lattice, a single diamond with a mass equal to 300,000 times the entire Earth! But don't count on getting rich mining this black gold ... the gravitational force on the surface of a white dwarf star is 300,000 times that of Earth, and you would weigh 30 million pounds there. Just enjoy the thought that billions of such diamonds really exist! The Blue Snowball planetary nebula (also known as NGC 7662), centered around a small, faint white dwarf. [NASA/HST, B. Balick] Blue Stragglers in M67 Direct N-body Modelling of Stellar Populations: Blue Stragglers in M67 Jarrod R. Hurley, Christopher A. Tout, Sverre J. Aarseth and Onno R.

Pols We present a state-of-the-art N-body code which includes a detailed treatment of stellar and binary evolution as well as the cluster dynamics. This code is ideal for investigating all aspects relating to the evolution of star clusters and their stellar populations. It is applicable to open and globular clusters of any age. We use the N-body code to model the blue straggler population of the old open cluster M67. Preliminary calculations with our binary population synthesis code show that binary evolution alone cannot explain the observed numbers or properties of the blue stragglers. On the other hand, our N-body model of M67 generates the required number of blue stragglers and provides formation paths for all the various types found in M67. This demonstrates the effectiveness of the cluster environment in modifying the nature of the stars it contains and highlights the importance of combining dynamics with stellar evolution. We also perform a series of N = 10 000 simulations in order to quantify the rate of escape of stars from a cluster subject to the Galactic tidal field.

The N-body simulation of M67 starts at an evolution age of 2 500 Myr and contains 5 000 binaries and 5 000 single stars initially. The Hertzsprung-Russell diagram and the spatial distribution in the XY-plane are shown at various epochs. In each figure main-sequence stars (black points), blue stragglers (filled blue circles), sub-giants and giants (filled red circles), naked helium stars (cyan points) and white dwarfs (magenta points) are distinguished. Binary stars are denoted by overlapping filled circles of colours appropriate to the stellar type of the components, with main-sequence binary components shown as green. Types of Stars Neutron Stars and Stellar Black Holes Introduction Despite what we may imagine, stars are not fixed and unchanging. Stars are formed, evolve and eventually exhaust their energy sources to end up as some type of stellar remnant. Although stars are not "alive", astronomers often talk in terms of a life cycle of a star and stellar evolution. Astronomers have developed a solid model for the key stages of stellar evolution though many details are still being tested and refined.

In this page we get an overview of the basic properties of the main types of stars, focusing on their sources of energy. This page is a condensed version of the stellar evolution section of the HSC Astrophysics option . Star Formation Stars form in dense molecular clouds within galaxies. These clouds of dust and gas obscure the early stages of stellar formation from optical telescopes. Fortunately recent advances in radio and infrared astronomy now allow astronomers to peer inside these clouds and gain a greater understanding of the processes involved in starbirth. Computationally intensive computer simulations also allow them to model the processes and test the results against observations. Gravity is the force responsible for stellar formation. Protostars form when sections of giant molecular clouds start to collapse. Clouds are initially diffuse enough that they do not contract unless something triggers an increase in the density of some regions within a cloud. Triggers for this initial collapse may include nearby supernovas or compression due to the motion of the arms in a spiral galaxy though other mechanisms are also possible. The gravitational collapse of a giant molecular cloud does not result in a single, massive star. Instead the cloud tends to fragment into smaller denser regions that each collapse to form star systems.

Up to a few thousand stars may typically form in a collapsing GMC resulting in an open (or galactic) cluster. The dense regions collapse due to gravitational attraction between the particles. Individual gas or dust particles move in towards the centre of the collapsing region, losing gravitational potential energy. As the total energy of the system is conserved the loss of gravitational energy is balanced by an increase in the kinetic energy of the particles. These particles then undergo more collisions which in turn raises the temperature of the gas. At this stage further collapse is only possible if the cloud can radiate away the thermal energy so that the radiation pressure outwards remains lower than the inward gravitational pull. This is achieved via convection cycling warm material upwards within the cloud, making the collapsing cloud visible in the infrared region. Credit: NASA , H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin ( STScI ), G. Hartig (STScI), the ACS Science Team, and ESA A star forming region in M 17, also known as the Omega or Swan Nebula, about 5,500 light years away in Sagittarius. For more details read the press release .

Continued collapse leads to higher densities so that eventually the cloud becomes opaque, trapping the thermal energy within the cloud. This then causes both the temperature and pressure to rise rapidly - the collapsing cloud is now a protostar. The timescale for this is basically a function of the mass of the collapsing cloud with more massive clouds collapsing more rapidly into a protostar. A 15 solar mass protostar may collapse in only 105 years whilst a star like our Sun would take around 50 million years. Low mass stars may take twice as long meaning that within a cluster the most massive stars have collapsed onto the main sequence, evolved off it and ended their "lives" before a 1/2 solar mass star has even made it onto the main sequence. The stages in the formation of a one solar mass star are: Initial collapse of a cloud causes it to heat up and become a protostar. Although cool it is very large, perhaps 20 &times the diameter of the Sun, thus its surface area is so great that its overall luminosity is very high, maybe 100 × its main sequence luminosity. As it radiates away energy, gravitational collapse pulls the protostar inwards rapidly.

Its temperature rises but this is offset by the decrease in size so that overall luminosity decreases significantly. Once the core temperature reaches 10 million K, coulombic repulsion between the now ionised hydrogen atoms (protons) is overcome and nuclear fusion commences. Hydrogen fuses to form helium nuclei, releasing energy in the process. Initially the increased outward radiation pressure is still insufficient to halt gravitational collapse but it does slow it down. The star's surface temperature increases significantly, compensating for the drop in size so that its luminosity increases slightly. The star's track moves up slightly and to the left on the H-R diagram over 10 million years. As the rate of core fusion increases due to higher core temperature, the outward gas and radiation pressures eventually match the inward gravitational force. The star attains a state of hydrostatic equilibrium and settles down onto the main sequence. This stage may take a few tens of million years. Main Sequence Stars NASA Our Sun, a main sequence star. Our Sun is a main sequence star. Other examples include α Cen, similar to the Sun, the cooler, redder and fainter Proxima Cen and hot, bright Achernar. What makes all these stars main sequence stars?

The answer is to be found deep inside such stars. The core temperature of main sequence stars is hot enough (> about 10 million K) that hydrogen nuclei (protons) can overcome coulombic repulsion and fuse together. The net result of this is that through several steps, hydrogen is fused to form helium nuclei. In the process a small amount of mass is converted into energy, released in the form of high-energy gamma photons. This hydrogen fusion (sometimes termed hydrogen burning although it is not a combustion process) provides the radiation pressure that supports main sequence stars against further gravitational collapse and ultimately is the source of energy fuelling life on Earth. Credit: CSIRO The proton-proton chain is the main hydrogen fusion sequence powering main sequence stars such as our Sun and those of lower mass. The net result is that four protons are fused to form a He-4 nucleus, gamma photons, positrons and neutrinos. The total mass of the products is slightly less than the constituents - the difference being converted to and released as energy. A different sequence, the CNO cycle (for carbon-nitrogen-oxygen) dominates in higher mass main sequence stars.

In the CNO cycle carbon-12 nuclei act as nuclear catalysts but the overall result is much the same as for the proton-proton chain, four protons are converted into a He-4 nucleus, releasing energy, primarily as high-energy gamma photons. The main factor that determines where a star lays on the main sequence is its mass. A star with a mass of about one-tenth that of the Sun has just enough gravitational force to heat the core to about 10 million K, the temperature needed for hydrogen fusion to start. If a protostar is less massive than this, fusion cannot be triggered and it becomes a brown dwarf or a "failed" star, emitting energy in the infrared. The greater the mass of a main sequence star, the higher its core temperature and the greater the rate of its hydrogen fusion. Higher-mass stars therefore produce more energy and are thus more luminous than lower mass ones. This comes at a cost though. High mass stars consume their core hydrogen fuel much faster than lower-mass ones. Our Sun has sufficient hydrogen in its core to last about 10 billion years (1010 years) on the main sequence.

A five solar-mass star would consume its core hydrogen in about 70 million years whilst an extremely massive star may only last three or four million years. Mass/MSun Key Properties of Main Sequence Stars Red Giants What happens when a main sequence star exhausts its stock of hydrogen within its core? Eventually the hydrogen fuel in the core runs out and fusion stops, shutting off the outward radiation pressure. Inward gravitational attraction causes the helium core to contract, converting gravitational potential energy into thermal energy. Although fusion is no longer taking place in the core, the rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star. This so-called shell-burning causes some interesting effects.The increased radiation pressure actually causes the outer layers of the star to expand to maintain the pressure gradient. As the gas expands it cools, just as a spray can feels colder after use as the gas has been released. This expansion and cooling causes the effective temperature to drop and the star thus appears redder. Convection transports the energy to the outer layers of the star from the shell-burning region.

The star's luminosity eventually increases by a factor of 1000 × or so. During this stage of expansion, the star will move up and to the right on the HR diagram along the Red Giant Branch (RGB). A G (V)-class star such as our Sun may end up as a high-K or low-M giant. Stars such as this are commonly referred to as red giants. When the Sun becomes a red giant it may have a radius 100 times larger than that it has as a main sequence star. The density of the outer layers of the red giant are extremely low. A star with the mass of our such Sun moves off the main sequence and up the red giant branch, fusing hydrogen into helium in hydrogen shell burning. A very short helium flash sees the start of helium core fusion and the star moves along the horizontal branch. Once the shell temperature is sufficient, helium shell burning starts and the star moves up into the asymptotic giant branch (AGB) on the HR diagram. Supergiants & Supernovae A. Dupree ( CfA ), R. Gilliland ( STScI ), FOC , HST , NASA The red supergiant Betelgeuse, imaged by the HST.

Supergiants such as Betelgeuse, Deneb, Rigel and Antares are some of the most prominent stars in our sky and visible over vast distances due to their extreme luminosities. These high-mass stars are rare and have very short lifespans relative to lower-mass stars. High-mass stars consume their core hydrogen at prodigious rates so may only survive on the main sequence for millions rather than billions of years. Once this fuel is used up, the core contracts due to gravity and heats up. This triggers helium-burning in the core. Unlike lower-mass stars, this helium fusion (triple-alpha process) starts gradually rather than in a helium flash. In moving off the main sequence, the effective temperature of the star drops as its outer layers expand. The decrease in temperature balances the increased radius so that the overall luminosity remains essentially constant. Energy liberated by helium fusion in the core raises the temperature of the surrounding hydrogen shell so that it too begins fusing. In time the core helium is used up resulting in further core collapse and gravitational heating. This then triggers carbon fusion to produce sodium, neon and magnesium.

Depending on the mass of the star, as each core fuel is used up further collapse leads to even higher temperatures that can trigger fusion of heavier elements. Through a combination of fusion and photodisintegration a range of heavier nuclei are formed up to iron for the most massive stars. The core region of a supergiant eventually resembles the layers of an onion with a dense iron core surrounding by shells of silicon and sulfur, oxygen and carbon, helium and an outer shell of hydrogen as shown in the diagram below. The final core temperature reaches about 7 × 109 K. The onion-like layers inside a supergiant in the final stages of its life. Successive layers correspond to the different elements produced by fusion, with a dense core of iron at the centre. Nucleosynthesis of elements above helium is less efficient so that each successive reaction produces less energy per unit mass of fuel. This means that the reactions occur at greater rates so that radiation pressure balances gravity. Whilst a massive star may spend a few million years on the main sequence, its helium core-burning phase may be a few hundred thousand years.

The carbon burning phase lasts a few hundred years, neon-burning phase a year, oxygen-burning half a year and the silicon-burning that produces iron takes only a day. In the last stages of a supergiant when the core temperature reaches 1010 K, iron can photodisintegrate, releasing gamma photons, protons and He nuclei. As iron is energetically the most stable nucleus, such reactions absorb rather than release energy and so the outward pressure drops. The core thus contracts further at speeds reaching tens of thousands of kilometres per second. The degenerate core electrons eventually have sufficient energy to react with the protons released by photodisintegration. This removes the electron degeneracy pressure so that core collapse continues. Once the temperature reaches 1012 K the neutrons become degenerate. Neutron degeneracy pressure actually stops the inward collapse of the core. The surrounding material is still collapsing, however, and slams into the now solid, rigid core and rebounds, forming a massive shockwave. This is similar to what would happen if you run headlong into a wall - you bounce back. In the collapsing massive star the energy involved is vastly greater than you running in to a wall.

A total of 1046 J is released in a matter of seconds, 50 × more than the Sun will release in 10 billion years on the main sequence. The massive star is ripped apart, becoming a Type II supernova. A Type II supernova reaches a luminosity of about 109 × solar luminosity at maximum brightness for a few days before fading over several weeks. It will outshine the combined luminosity of all the other stars in its spiral galaxy for a few weeks. Stars about 20 × the mass of our Sun may form a Type II supernova. In 1987 a massive star in the Large Magellanic Cloud, a satellite galaxy of our own Milky Way, became a supernova. This star, SN1987A, was the first supernova visible to the unaided eye in over 300 years. After and before images of SN1987A in the Large Magellanic Cloud. During the final destruction of the star in a supernova explosion, a high flux of neutrons is released as iron nuclei are ripped apart.

These neutrons can be captured by many of the heavy nuclei to produce other nuclei in a method known as the r-process (r for rapid). The high number of neutrons available in these few seconds can be captured by unstable nuclei before the nuclei have had a chance to decay. In this way nuclei of elements such as lead, gold and all the way up to uranium can be synthesised. The gold in jewellery here on Earth was in fact formed during the fiery last seconds of a massive star. The destructive event not only creates new elements but provides the mechanism by which they are dispersed into space as a supernova remnant (SNR). White Dwarfs Credit: NASA , H. E. Bond and E. Nelan ( Space Telescope Science Institute , Baltimore, Md.); M. Barstow and M. Burleigh (University of Leicester, U.K.); and J. B. Holberg (University of Arizona) The white dwarf, Sirius B (the small dot at lower left) and its main sequence companion, Sirius A. Stars with masses similar to our Sun will end up as white dwarfs. These stellar remnants have unusual properties. Firstly, they are very small but the more massive white dwarfs are actually smaller than less massive ones.

With their fuel used up no fusion takes place so there is no outward radiation pressure to withstand gravitational collapse. More massive stellar cores experience stronger gravitational force so actually compress more. A 0.5 solar-mass white dwarf has a radius 1.9 × that of Earth, a 1.0 solar-mass one is only 1.5 earth radii whilst a 1.3 solar-mass white dwarf 1.4 earth radii. A white dwarf is composed of carbon and oxygen ions mixed in with a sea of degenerate electrons. It is the degeneracy pressure provided by the electrons that prevents further collapse. A white dwarf, with a mass roughly that of the Sun packed into a volume not much greater than the Earth must have an extremely high density. At 109 kg m-3 its density is one million times greater than that of water. Although its surface temperature is about 10,000 K, the core temperature may be as high as 107 K. The heat trapped within a white dwarf will gradually be radiated away by it but with its small radius, a white dwarf has only a small surface area. Heat therefore cannot escape quickly.

In fact it will take tens to hundreds of billions of years for a white dwarf to radiate away its heat and cool down to a black, inert clump of carbon and degenerate electrons. As the Universe is not yet old enough for this to have happened, all the white dwarfs that have ever formed in single-star systems are still white dwarfs. Typical luminosities are less than 10-3 that of our Sun. More massive white dwarfs, having smaller surface areas but more trapped heat, take longer to cool down than lower-mass ones. As white dwarfs are so faint they are also hard to detect. We are only able to observe relatively close ones. Nonetheless white dwarfs are thought to comprise about 10% of the stars in our galaxy. Nearby examples are Sirius B and Procyon B, both of which are found in binary systems. Neutron Stars and Stellar Black Holes A massive star undergoing core collapse produces a Type II supernova. What happens to the core material that is not ejected depends on its mass. Photodisintegration of iron in the last stages of the massive star's life releases protons that in turn react with electrons to form neutrons.

These neutrons combine with existing core material to form neutron degenerate matter. If the remaining mass of the star is less than about 3 solar masses the collapse of the core is halted by the degeneracy pressure of the neutrons. The result is one of the most intriguing types of objects in the Universe, a neutron star. Neutron stars are composed of degenerate neutron matter with a density about that of atomic nuclei, ~ 1017 kg. m-3. A thimble-full of this material has a mass of almost 109 tonnes. They range in mass from a lower value equal to the Chandrasekhar Limit of 1.4 solar masses up to about 3 solar masses. This upper limit is not well-defined and may be up to 5 solar masses in some models. A neutron star is typically about 10 km across. We thus have a very exotic object with twice the mass of the Sun packed into a sphere the size of a small city! Due to the conservation of angular momentum, a neutron star spins at a high rate. This is analogous to an ice skater spinning faster as they draw their arms in close to their body.

Whereas a star such as the Sun rotates on its axis roughly once a month, a neutron star can rotate dozens of hundreds of times a second. Credit: NASA/ Chandra Sometimes though a star is so massive that the mass of the material left after all other mass-loss processes exceeds the limit that even neutron degeneracy pressure can withstand. At this stage then the material keeps collapsing inwards until all the mass becomes concentrated at a single point, a singularity. It is now a black hole. Black holes are even more exotic objects than neutron stars. With all the mass concentrated at a point they have extremely high gravitational fields. They are referred to as black because not even light can escape from them once it has crossed a region known as the event horizon. At the event horizon, the escape velocity equals the speed of light, c. Black holes are therefore hard to observe because they do not emit light at any waveband. Rather than look for a black hole itself, astronomers infer their presence due to their effect on surrounding matter.

Secrets of Vampire Stars Revealed in Hubble Telescope Data Secrets of Vampire Stars Revealed in Hubble Telescope Data By Sarah Lewin, Staff Writer | December 8, 2015 05:19pm ET MORE This artist's illustration depicts the birth of a blue straggler star as it siphons off material from a companion red giant star. Credit: NASA/ESA, A. Feild (STScI) New observations from the Hubble Space Telescope have revealed the hidden hosts feeding vampirelike blue straggler stars, strange objects that appear to stay uncannily young-looking instead of growing into old red giant stars. When an aging star exhausts most of its fuel, it balloons outward into a huge red giant . But in a cluster of stars that formed at the same time, some will look eerily young: While the stars' contemporaries are bulky and cooler, they remain trim with a hotter, bluer light as if they're getting infusions of new fuel. So blue straggler stars get their name because they "straggle" behind the typical star life cycle.

A group of astronomers surveyed 21 blue straggler stars in a cluster about 5,000 light-years away to determine their youthful secrets. The Hubble Space Telescope was able to spot evidence of orbiting white dwarf stars that contributed mass to many of the stragglers. [ Amazing Hubble Space Telescope Photos: Latest Views ] Scientists have known about these blue stragglers since 1953, but the source of their extra fuel has remained a mystery. Researchers suspected they might be in a binary system — two stars which orbit each other closely — where one star siphons material off its partner star. But the mechanism wasn't certain. They could also have merged with other stars or collided with unlucky victims. A blue straggler star forms by siphoning the mass from a red giant star it closely orbits in a binary system. In the second frame, the bright, hot white straggler orbits the remains of the first star, now a faint white dwarf. Credit: NASA/ESA, A.

Field (STScI) A 2011 study surveyed the star cluster NGC 188 to investigate its blue straggler population , and now follow-up observations from the Hubble Space Telescope have revealed the ultraviolet signatures of white dwarf stars locked into orbit with seven of the blue stragglers. An additional seven show other evidence of mass transferring from another star, although white dwarfs haven't been spotted. "Until now there was no concrete observational proof, only suggestive results," Natalie Gosnell, an astronomer at the University of Texas, Austin, and lead author of the new work, said in a statement . "It's the first time we can place limits on the fraction of blue stragglers formed through mass transfer." For about two-thirds of the blue stragglers surveyed, this research confirms this mass transfer process. The more massive star in a binary system swells into a red giant, overshadowing its companion — but then, its mass is siphoned away by the companion star. The balance shifts as the companion star glows hotter and brighter with the extra mass until only the small, dense stellar core remains of the first star — it collapses into a white dwarf.

Viewers from Earth see only the blue straggler as a single, unusually bright and hot star. The researchers sniffed out the white dwarfs' presence by detecting the stragglers' movement back and forth, caused by orbiting with another star, and verified it by identifying a bright ultraviolet signal with Hubble. Only signatures from the youngest, hottest white dwarfs can be detected that way, but the binary systems found will add to researchers' understanding of how stars grow and change in such systems. Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz. Hydrodynamic simulation of a white dwarf-main-sequence star collision in a globu 1986ApJ...306..543S Abstract Several hundred physical collisions occur in the core of a typical globular cluster during a Hubble time. The most common types of collision involve stars of large cross section (giants) and/or very common stars (main- sequence and degenerate dwarfs). Any collision with a red giant is likely to disperse that star's envelope and leave it as an ordinary white dwarf.

Two colliding main-sequence stars (MSS) are likely to coalesce and may appear as He-rich blue stragglers. Colliding MS and degenerate stars have been predicted to form "massive disks" around the degenerate stars. We have carried out a two-dimensional hydrodynamic simulation (including the effects of nuclear reactions) of a white dwarf-MSS head-on collision (with collision parameters appropriate to the core of a globular cluster). Accretion onto the white dwarf is rapidly followed by dynamical expansion of the MSS, powered by nuclear reactions' heating and redistribution of the MSS kinetic energy. Most or all of the MSS is disrupted during the simulated collision, which liberates more than 1048 ergs of nuclear energy (i.e., the MSS binding energy) in ˜ 1000 s. The angular momentum in a collision with impact parameter comparable to RMSS may prevent this fate and permit heavy disks to form. The effects of nuclear reactions' heating must be included in future studies (hopefully including three-dimensional hydrodynamic simulations) to lend more credence to the massive disk hypothesis. Printing Options

Question: White dwarfs, red giants, and blue stragglers are all types of what?
Answer: Stars